In conjunction with economic development, new development of natural resources, such as coal or the like, is progressing. Specifically, mining is underway at regions with a severe natural environment which have not so far been developed. Accordingly, the track environment is becoming remarkably severe in overseas freight railways used to transport natural resources. There is a demand for rails to have toughness or the like in regions with cold weather in addition to higher wear resistance than ever. In such circumstances, there is a demand to develop rails having higher wear resistance and higher toughness than those of presently-used high-strength rails.
In general, it is known that the refinement of a pearlite structure, specifically, grain refining in an austenite structure which is yet to be transformed into pearlite or the refinement of pearlite blocks is effective to improve the toughness of a pearlite steel. In order to achieve grain refining in an austenite structure, during a hot rolling, the rolling temperature is decreased and the rolling reduction rate is increased and, furthermore, a heat treatment by low-temperature reheating after the hot rolling of rails is implemented. In addition, in order to achieve the refinement of a pearlite structure, pearlite transformation starting from the inside of austenite grains is accelerated by utilizing transformation nuclei or the like.
However, in the manufacturing of rails, from the viewpoint of ensuring formability during the hot rolling, there are limitations on a decrease in the rolling temperature and an increase in the rolling reduction rate; and thereby, sufficient refinement of austenite grains could not be achieved. In addition, with regard to the pearlite transformation from the inside of austenite grains by utilizing transformation nuclei, there are problems in that the amount of transformation nuclei is difficult to control, and the pearlite transformation from the inside of grains is not stable; and thereby, sufficient refinement of a pearlite structure could not be achieved.
Due to these problems, a method has been applied to fundamentally improve the toughness of rails having a pearlite structure in which low-temperature reheating is conducted after hot rolling a rail, and then pearlite transformation is performed by accelerated cooling so as to refine a pearlite structure. However, recently, rails have been made to include a high content of carbon for improving the wear resistance; and therefore, there is a problem in that coarse carbides remain inside austenite grains during the above-described low-temperature reheating treatment, which lowers the ductility and toughness of a pearlite structure after the accelerated cooling. In addition, since this method includes reheating, there is another problem in regard to economic efficiency, such as a high manufacturing cost, a low productivity or the like.
Consequently, there is a demand to develop a method for manufacturing a high-carbon steel rail that ensures the formability during hot rolling and refines the pearlite structure after hot rolling. In order to solve this problem, methods for manufacturing a high-carbon steel rail shown below have been developed. The major characteristics of those methods for manufacturing a rail are that the following finding is utilized so as to refine the pearlite structure; and the finding is that austenite grains in a high-carbon steel are easily recrystallized at relatively low temperatures and even with a small rolling reduction rate. As a result, fine grains with similar grain diameters are obtained by continuous rolling under a small rolling reduction rate; and thereby, the ductility and toughness of a pearlite steel is improved (for example, Patent Documents 1, 2 and 3).
In a technology disclosed by Patent Document 1, 3 or more continual passes of rolling are conducted with a predetermined interval of time in the finish rolling of a high carbon steel rail; and thereby, a rail having high ductile can be provided.
In a technology disclosed by Patent Document 2, two or more continual passes of rolling are conducted with a predetermined interval of time in the finish rolling of a high carbon steel rail, and furthermore, accelerated cooling is conducted after the continuous rolling. As a result, a rail having superior wear resistance and high toughness can be provided.
In a technology disclosed by Patent Document 3, cooling is conducted between passes of rolling in the finish rolling of a high-carbon steel rail, and conducting accelerated cooling is conducted after the continuous rolling. As a result, a rail having superior wear resistance and high toughness can be provided.
The technologies disclosed by Patent Documents 1 to 3 can achieve the refinement of an austenite structure at a certain level and exhibit a slight improvement in toughness by the combination of the temperature, the number of rolling passes, and the interval of time between passes during the continuous hot rolling. However, there is a problem in that these technologies do not exhibit any effects in regard to fracture starting from inclusions present inside the steel; and thereby, the toughness is not fundamentally improved.
Considering these circumstances, the addition of Ca, the reduction of the oxygen content, and the reduction of the Al content have been studied in order to suppress the generation of typical inclusions in rails, that is, MnS or Al2O3. The characteristics of these manufacturing methods are that MnS is changed to CaS by adding Ca in the preliminary treatment of hot metal so as to become harmless, and furthermore, the oxygen content is reduced as much as possible by adding deoxidizing elements or applying a vacuum treatment so as to reduce the amount of inclusions in molten steel, and technologies of which have been studied (for example, Patent Documents 4, 5 and 6).
The technology in Patent Document 4 discloses a method for manufacturing a high-carbon silicon-killed high-cleanliness molten steel in which the added amount of Ca is optimized to fix S as CaS; and thereby, the amount of elongated MnS-based inclusions is reduced. In this technology, S which segregates and concentrates in a solidification process reacts with Ca which similarly segregates and concentrates or calcium silicate generated in the molten steel; and thereby, S is sequentially fixed as CaS. As a result, the generation of elongated MnS inclusions is suppressed.
The technology in Patent Document 5 discloses a method for manufacturing a high-carbon high-cleanliness molten steel in which the amount of MnO inclusions is reduced; and thereby, the amount of elongated MnS inclusions precipitated from MnO is reduced. In this technology, a steel is tapped in a non-deoxidized or weakly deoxidized state after being melted in an atmosphere refining furnace, and then a vacuum treatment is conducted at a degree of vacuum of 1 Torr or less so as to make the dissolved oxygen content be in a range of 30 ppm or less. Next, Al and Si are added, and then Mn is added. Thereby, the number of secondary deoxidization products is reduced which will become crystallization nuclei of MnS that crystallizes out in finally solidified portions, and the concentration of MnO in oxides is lowered. Thereby, the crystallization of MnS is suppressed.
The technology in Patent Document 6 discloses a method for manufacturing a high-carbon high-cleanliness molten steel with reduced amounts of oxygen and Al in the molten steel. In this technology, a rail having superior damage resistance can be manufactured by limiting the total amount of oxygen based on the relationship between the total oxygen value in oxide-based inclusions and the damage property. Furthermore, the damage resistance of rails can be further improved by limiting the amount of solid-soluted Al or the composition of inclusions in a preferable range.
The above-described technologies disclosed in Patent Documents 4 to 6 control the configurations and amounts of MnS and Al-based inclusions generated in a bloom stage. However, the configuration of inclusions is altered during hot rolling in the rolling of rails. In particular, Mn sulfide-based inclusions elongated in the longitudinal direction by rolling act as the starting points of fracture in rails; and therefore, there is a problem in that the toughness of rails cannot be stably improved in the case where only the inclusions in the bloom stage is controlled.
From such circumstances, it has become desirable to provide a pearlitic rail having superior wear resistance and toughness in which both the wear resistance and toughness of a pearlite structure are improved.